The present invention relates to the treatment of a gaseous medium laden with particles and in particular of solid, liquid or gaseous pollutants or impurities contained in a gaseous medium, such as the exhaust gases from an internal combustion engine.
A particular, but not exclusive, application is purification of the exhaust gases of a diesel engine.
The pollutants issuing from exhaust systems include:                carbon compounds: CO, CO2;        nitrogen compounds: NO, NO2 (usually called nitrogen oxides NOx) etc.,        organic compounds, such as hydrocarbons (HC) etc.;        sulphur compounds: SO2, SO3, etc.;        organic particles;        etc.        
Emissions of organic particles are especially characteristic of diesel engines and are composed of a carbon-containing material (soot), on which various organic species are adsorbed (SOF: Soluble Organic Fraction).
Numerous methods and devices for treating the exhaust gases from an internal combustion engine have already been proposed previously.
In particular, the use of oxidation catalysts on a particulate support or a monolithic support is known, especially for oxidizing CO and unburnt hydrocarbons.
For the particles from diesel engines, there are also trapping systems that can be regenerated.
Devices for treating gases employing corona-effect electrostatic filters are also known, in particular from documents EP-A-0299 197 (U.S. Pat. No. 4,871,515) and U.S. Pat. No. 4,478,613.
The devices in these two documents operate according to different principles. Thus, in the case of the device to which the first of these two documents relates, the particles are to be trapped in a collecting structure, whereas with the device described in the second of these two documents the particles form agglomerates on the collecting structure which are then detached from this collecting surface and are entrained by the stream of gases circulating in the device, before being separated from the latter by means of a mechanical separator.
The invention aims to improve the known treatment devices, in particular with respect to their efficiency.
It also aims to provide a treatment device that is compact, inexpensive and easy to manufacture.
To this end, it proposes a device for treating a particle-laden gaseous medium, having at least one corona-effect electrostatic filter comprising:                a longitudinal casing;        a longitudinal channel for the gases, extending in the casing and with its two opposite ends adjacent to the gas inlet and outlet of the electrostatic filter, respectively;        an emitting structure extending longitudinally and roughly at the centre of the channel; and        a collecting structure extending longitudinally between the channel and the casing and comprising a plurality of cavities forming sites for trapping the particles contained in the gaseous medium;        characterized in thatthe emitting structure comprises a plurality of serrated plates arranged transversely to the longitudinal direction of the channel.        
A treatment device of this kind meets the requirements that have just been mentioned. In particular, this device proves to be especially efficient in terms of collection of particles, as will be described in more detail later.
For reasons of efficiency of collection and ease of manufacture, the serrated plates are constituted of stars that are to be connected to a circuit supplying a stabilized high voltage (several kV).
A washer with a star-shaped central recess might, for example, also be suitable.
Other solid or perforated geometric shapes preferably having a plurality of vertices directed towards the collecting structure can be arranged between these stars. These geometric shapes can, for example, consist of washers or rings perforated with holes of various diameters.
A possible form of the circuit supplying a stabilized high voltage consists of providing a converter or transformer supplying a voltage between 0 and 15 kV controlled by a regulator.
Preferably, the voltage applied is negative and greater than about 6 kV.
Also for reasons of efficiency, the collecting structure preferably includes a separator or mat-type eliminator made from metal wire fabric.
According to the preferred embodiment, the metallic fabric has a herringbone structure that facilitates penetration of the particles into the fabric.
As a variant, it will also be possible to employ, for example, a collecting structure provided with grooves, channels, flutes etc.
According to the preferred embodiment, the separator is of cylindrical shape and surrounds the serrated plates of the emitting structure, aligned on the axis of the cylindrical shape of the collecting structure.
Advantageously, in this case, the emitting structure and the collecting structure are mounted on a supporting structure, with which they form a removable filter cartridge of the treatment device.
In the case of a treatment device in which the gas inlet and outlet extend transversely to the longitudinal channel for the gases, the serrated plates are, preferably, carried by a rod connected to the circuit supplying a high voltage and which is carried, at each of its ends, by an insulator protected by a bell-shaped cover (bell).
To increase the efficiency of collection, the treatment device can, advantageously, have a second electrostatic filter, original in itself, and having metal stars carried by one face of a perforated metal disk connected to the circuit supplying a stabilized high voltage and mounted upstream of a separator of cylindrical shape, made from a metal wire fabric.
For treating the gaseous pollutants, the treatment device preferably also has an oxidation catalyst with a monolithic support upstream of the electrostatic filter or filters.
This treatment device can also include a mechanical filter upstream of the electrostatic filter or filters and, if appropriate, of the oxidation catalyst, for example for retaining oily emulsions by using a devesiculating filter, for example of the inverted-V impact type.
In accordance with a configuration that is original in itself, the mechanical filter comprises a metal mesh filter, i.e. it is made from a metal wire fabric or metallic fabric, defining a forced channel for the gaseous medium entering the treatment device and associated with an electrical resistance for raising the temperature of the gaseous medium.
This filtering structure makes it possible to raise the temperature of the gaseous medium to the working temperature of the oxidation catalyst. In particular, however, it provides a treatment device that is especially compact, by causing combustion of particles retained in the filter. The result of this is that a smaller quantity of particles has to be treated by the electrostatic filter or filters, and it is therefore possible to reduce the size of the treatment device.
The said treatment device can also be provided with an inlet for air for oxidation and/or an inlet for air for cleaning.
To combat the backpressure effects that adversely affect the operation of an internal combustion engine and are associated with a device of this type, the latter can also be provided with aspirating means downstream of the electrostatic filter or filters.
In the preferred embodiment, the treatment device has, in addition, at least one cylindrical casing for housing the electrostatic filter or filters and, if appropriate, the oxidation catalyst and/or the mechanical filter.
The present invention relates, finally, to a vehicle equipped with a treatment device as defined above.
Other objects, characteristics and advantages of the present invention will become clear from the description given below, which refers to the appended drawings in which: